Continuous-wave (CW) lasing was recently achieved in colloidal quantum dots (CQDs) by lowering the threshold through the introduction of biaxial strain. However, the CW laser threshold is still much higher than the femtosecond threshold. This must be addressed before electrically injected lasing can be realized. Here we investigate the relationship between threshold and temperature and find a subpicosecond recombination process that proceeds very efficiently at temperatures reached during CW excitation. We combine density functional theory and molecular dynamics simulations to explore potential candidates for such a process, and find that crystal defects having thermally vibrating energy levels can become electronic traps-i.e., they can protrude into the bandgap-when they are sufficiently distorted at higher temperatures. We find that biaxially strained CQDs, which have a lower femtosecond laser threshold than traditional CQDs, result in less heat for a given transparency/gain level and thus undergo this trapping to a lower extent. We also propose methods to tailor CQDs to avoid self-compensating defect traps.
Keywords: CdSe; amplified spontaneous emission; colloidal quantum dots; doping; optical gain; temperature dependence; trapping.